FIG. 4a shows a cross-sectional view of a prior art semiconductor laser device conducting a single mode oscillation, which is disclosed in Applied Physics Letters 49(18), Nov. 3, 1986, pages 1148-1150. FIG. 4b shows a cross-sectional view taken along line B-B' of FIG. 4a. The laser includes an n type InP substrate 11. An p type InP current blocking layer 3 is disposed on substrate 11. A n type InP current blocking layer 2 is disposed on blocking layer 3. These blocking layers 2 and 3 are discontinuous, being separated by V-shaped groove 20. An n type InP cladding layer 9 is buried in the V-shaped groove 20. An InGaAsP active layer 6 is disposed on cladding layer 9. A p type InP cladding layer 4 is disposed on active layer 6 and blocking layers 2 and 3. A diffraction grating 8 is produced on p type InP cladding layer 4 opposite the V-shaped groove. A p type InGaAsP contact layer 12 is disposed on the cladding layer 4. (Buried Crescent) or V-shaped groove structures are suitable for achieving the desirable characteristics of semiconductor lasers, such as low threshold current, high power output, and fundamental transverse mode oscillation, the diffraction grating cannot be produced in the groove. Therefore, in this prior art laser device, after producing a semiconductor laser of the Fabry-Perot type having a V-shaped groove structure by liquid phase epitaxy, diffraction grating 8 is produced on the last epitaxially grown p type InP cladding layer 4 by an electron beam deposition method. In this structure, the distance between the diffraction grating 8 and active layer 6 is 0.7 micron.
The diffraction grating and active layer must be close to each other to achieve single longitudinal mode oscillation. The optimum distance between them may be about 0.2 micron. However, the above described structure of the prior art semiconductor laser does not meet this condition. Since the diffraction grating is produced on the last epitaxially grown layer (p-InP cladding layer 4), the diffraction grating is located far from the active layer, making single longitudinal mode oscillation difficult. Therefore, a secondary diffraction grating 8 (having period of 0.46 micron) of 0.3 micron height is utilized here.